1. Field of the Invention
The present invention refers to a protection device for driving devices for electric motors and corresponding driving device.
2. Description of the Related Art
In the prior art, driving devices for three-phase motors are known that implement the operating phase of the motor brake, i.e., the operating phase in which the three-phase motor is stopped rapidly.
FIG. 1 shows a diagram of a three-phase motor provided with a driving device that implements the motor brake operating phase. The three-phase motor comprises the star-connected windings 1-3 (the solution is also suitable in the case of delta-connected windings) at the common terminal D and having the other terminals connected respectively at the nodes A-C with the half-bridges 4-6; the latter are made with power switches such as MOS transistors or bipolar power switches or IGBTs and are the power stage for driving the windings 1-3. The half-bridges 4-6 are connected between a supply voltage Vs and ground GND and are driven by respective terminal elements 7-9 forming part of a driving device 10. The elements 7-9 are circuitally connected to a main element 11 that implements several control functions including the motor brake or brake operating phase. To obtain rapid braking of the electric motor the windings 1-3 are short-circuited to the supply voltage Vs or to ground GND; the current in the windings of the motor will reach a zero value in a period of time that depends on the features of the motor and on the application.
During actuation of the brake operating phase in the windings of the motor current overshoots occur due to the presence of the back electromotive force (BEMF) that depends on the electric constant of the motor and on the rotation speed. Said overshoots can exceed the maximum permitted current in the half-bridges 4-6 and can determine failure of the brake operating phase or also failure of the power stage formed by the half-bridges.